1. Field of the Invention
The present invention relates to a method for controlling the ejection of ink drops from an inkjet printhead comprising an array of ink chambers with nozzles arranged for applying a row of ink dots on a receiving medium using a control signal. The invention further relates to an inkjet print system for printing images on a receiving medium by controlling a printhead to eject ink.
2. Description of the Related Art
Inkjet print systems are widely known and applied to print images on a variety of receiving materials, such as paper, cardboard, vinyl, or any other substantially flat material by an inkjet process. In these systems, all kinds of ink or other marking material are used, either water, solvent, UV-curable or phase-change based, which are all referred to as inks in this description. Drops of ink are made by first increasing a pressure in an ink chamber, either by partly vaporizing the ink in the chamber using a heating element or by generating an acoustic wave in the ink chamber by a piezo-electric actuator or otherwise, thereby pushing ink through a nozzle, which is in connection with the ink chamber, and then decreasing the pressure to stop a flow of ink through the nozzle. In the case of acoustic actuation, a reverse operation by first decreasing a pressure for filling an ink chamber with ink and then increasing the ink chamber pressure is also possible and often used. Upon hitting the receiving material, an ink drop causes an ink dot for locally marking the material by changing an optical density. The size of the dot depends on several factors, among which the size, or volume, and velocity of the ink drop and the mutual interaction between the ink and the receiving material. An array of ink chambers with nozzles is provided in a printhead to be able to print a row of ink dots and a movement of the printhead relative to the receiving material in a direction perpendicular to the row of ink dots enables the application of ink drops to an area of the receiving material. These inkjet systems are known as drop on demand systems.
A control signal is provided to the inkjet printhead to determine a timing of a drop ejection and the drop properties, such as the drop size and drop velocity. Some printheads only allow one drop size, but more and more printheads may be controlled to apply several drop sizes by each of the nozzles in the array. The determination of the control signals for generating a printed image from a digital image, wherein a local color of pixels is specified, involves a number of elaborate calculations that take into account among others the arrangement of the nozzles in the printhead, the movement of the printhead relative to the receiving medium and the velocity and size of the ink drops. Some of these characteristics may need calibration for achieving optimal performance.
Inkjet printheads comprise more and more nozzles, on the one hand to increase a printed image resolution in terms of dots per inch, on the other hand to increase a width of a printhead for printing a larger swath while crossing a receiving medium. Occasionally, a nozzle is unable to produce an ink drop as required according to specifications for a regular ink drop. This may be caused by an incorporation of a gas bubble in the ink chamber or another obstruction that hinders a regular formation of an ink drop. In some cases, a blockade is only temporary and a period of not activating the nozzle will resolve the impairment. In other cases, a non-functioning nozzle is the result of production tolerances or aging of the printhead. In any case, an individual nozzle failure does not necessarily mean the end of life of a printhead. According to known prior art, a control signal may be distributed to neighboring nozzles in an attempt to obfuscate a missing dot. The control signal may partly or completely be passed to a neighboring position within the array of nozzles in order to preserve an intended change of optical density using one and the same colorant, albeit on a slightly different position than specified by the digital image. In another version, a control signal is passed to a nozzle associated with the same position on the receiving material as the anomalous nozzle, but ejecting an ink drop of a different colorant. A further method for compensating a non-functioning nozzle involves the application of a second nozzle that passes the same position in a print strategy that applies several passes of the printhead over an area of the receiving material. The second nozzle thus prints additional ink in order to compensate the missing ink from the blocked nozzle at the cost of a slower print speed. It is noted that the functioning of a nozzle depends on a precision of the specifications for a regular ink drop. In some print systems, it is advantageous to use narrow specifications, since it allows a nozzle to recover during a time it is not activated by a control signal.
Whether a nozzle produces an ink drop according to specifications may be determined either by measuring drop properties, such as velocity and volume, or by measuring a position and size of an ink dot that is associated with an ink drop and a nozzle that has produced the drop. Commonly, optical methods are employed for this purpose, but also known is to use a feedback signal from a sensor element associated with a nozzle. Either way, a nozzle may be labeled as functioning and will be activated in accordance with an image to be printed. A non-functional nozzle is not activated, since the resulting dot will not be according to the specifications and the non-activation may restore its functioning. The defect in the printed image corresponding to a non-functional nozzle may have a limited impact on the print quality, if the size of the ink dots is sufficiently large, thereby already obfuscating missing dots, or if a compensation method is applied as sketched above.
However, there remains a problem in print quality in ink jet print systems from occasional lines of missing ink dots, related to a single nozzle. An object of the present invention is to further improve print quality regarding these lines.